Mathematical models of activation and recovery in both two-dimensional and three-dimensional cardiac syncytical tissue are being formulated and the electrical sources and their fields determined. The models are based on both known histology and measured electrical parameters of cardiac tissue, including intracellular and interstitial anisotropy. Results will be checked against experiments conducted here and available in the literature as well as against computer models which incorporaate active (e.g., Hodgkin-Huxley) elements. Additional topics include study of the inverse problem in electrocardiography, particularly the transformation of body surface potentials to the epicardium. In this connection, a study of the effect of the inhomogeneities and geometry on the relationship between epicardial and body surface potentials is being considered.